The first known observation of the shape memory transformation was by Chang and Read in 1932. They noted the reversibility of the α solid phase transformation in a gold cadmium (AuCd) alloy from metallographic observations and resistivity change measurements. In 1951 the shape memory effect (SME) itself was observed in a bent bar of AuCd. Then in 1962 Buehler et al. discovered the SME effect in nickel-titanium (NiTi). The group named the alloy “Nitinol”, after its elemental components and place of origin. The “Ni” and “Ti” are the atomic symbols for nickel and titanium, respectively, the “NOL” stands for the Naval Ordinance Laboratory.
The first use of an SMA in a heat engine application, in which thermal energy is changed into mechanical work, was accomplished in 1973 by Ridgway Banks and Hap Hagopian of the Lawrence Berkely Laboratory at the University of California. SMA heat engines became very popular as fascinating visual demonstrations of the shape memory effect and of heat engines in general. However, practical engineering applications were not successful, as the theoretical upper limit of efficiency (Carnot efficiency) approached only 4-5%. Furthermore, most heat engine designs were extremely complex.
Microelectromechanical systems (MEMS) researchers are always looking for new designs, methods and materials, especially as the demand for silicon micromachined devices continues to soar in a variety of fields including medicine, biotechnology, the semiconductor industry and a host of other applications. With the development of thin film fabrication techniques in the last decade, SMA thin films have attracted great interest as a potentially powerful actuation material for MEMS. This is mainly due to the fact that SMA thin films are capable of large forces and displacements compared to other actuator types such as electrostatic, electromagnetic, and piezoelectric actuators.
The possibility of integrating NiTi SMA thin films into a silicon micromachining process was first demonstrated in 1990, with the first SMA actuated microvalve reported in 1992. Since then, researchers have worked extensively to clarify the properties of SMA thin films, while simultaneously attempting to fabricate other micro devices driven by SMA thin films. Although a number of other NiTi actuated microdevices, including microvalves, micropumps, microrelays, micromirrors and out-of-plane spacers, have been reported recently, the TiNi Alloy Co. microvalve is currently the only known commercially available device.
Recently researchers have been developing MEMS based internal combustion engines and turbines as possible replacements for batteries. The major benefit comes in the form of the high energy density associated with fuels when compared to electrochemical cells. However, shape memory alloy based MEMS heat engines have not been mentioned or even conceived of prior to the current invention by the inventors. The prior art does mention the use of NiTinol materials in heat engine designs nor in MEMS applications; no one has even hinted at combining these two applications of shape memory alloys. This is likely due to the extremely complicated designs required by prior SMA heat engines used as visual demonstrations.
In addition to the energy density advantage MEMS heat engines offer over other energy sources, MEMS heat engines offer the possibly of exploiting favorable scalings of several physical quantities. Scaling normal sized devices down to the microscale can shift the influence of certain physical parameters on the total system dramatically. For example, the ratio of surface area to volume is significantly larger at the microscale, making surface tension, an effect routinely ignored at the macroscale, a dominant feature of microfluidic devices. Such favorable scalings provide for fast heat transfer rates and large temperature gradients for an SMA-MEMS heat engine, features which are absent in macro-sized SMA heat engines and therefore significantly hinder their performance and efficiency. Furthermore, SMA thin films produced in batch fabrication thin film processes may have different material properties compared to bulk materials, most notably extrinsic stresses induced by high temperature processing and deposition. Harnessing this stress offers the possibility of greatly simplifying the fabrication process of an SMA-MEMS heat engine.
In summary, the prior art MEMS engines suffer from large losses through friction and other losses to the point where they nearly have difficulty producing more energy than they require to operate. SMA-macro engines lack sufficient thermal gradients, speed, efficiency and adequate harnessing mechanisms for operation.